1. Field of the Invention
This invention relates generally to oil wells and more particularly to means for shielding a subsea wellhead and for mitigating impact loads applied thereto.
2. Description of the Prior Art
In the art of recovering oil from a subterraneous formation, various methods of geophysical exploration are employed. In the drilling of a well, either of two common systems may be employed. The first system utilizes cable tool drilling in which a heavy drilling bit which may be many feet in length is moved up and then is allowed to drop so as to produce the hole by displacement. The cuttings are then bailed out after periodical sinking of the hole. In a second or "rotary" system, an enlarged bit at the end of a drill stem or drill pipe is rotated while it is let down in the hole to be formed. A drilling fluid is pumped down in the drill pipe. The drilling fluid is passed through holes or eyes in the bit and then rises outside of the drill pipe to the surface carrying the cuttings with it. These cuttings are then separated from the "mud" or fluid by gravity separation along a settling ditch or passed over a screen or both. The drilling fluid then proceeds to a sump for recirculation.
In the cable tool system described hereinabove a drilling fluid may also be employed. The drilling fluid performs a number of functions such as carrying the cuttings to the surface as described above and also lining the bore hole by a filtering action of the liquid part of the fluid in the bore wall. Where an aqueous fluid is employed in order to give it body, various solids may be used. In order impart specific gravity to the drilling fluid, a heavy material such as barytes may be used. To control the viscosity of the drilling fluid a polyphosphate may be used. When the liquid part of the drilling fluid is oil, such as fuel oil, various viscosity imparting agents and barytes may be employed.
During the course of drilling, the bore hole is cased at various stages in the sinking of the well. The casing operation is performed by a steel casing being let down into the hole. The steel casing is cemented in place to shut off water formations and to prevent caving. There may be a number of "strings" of casings in a well.
To complete the well for production, the drilling fluid is bailed out of the well leaving a coating of clay, etc. on the bore walls which then must be removed at the producing formation. To accomplish this the well may be "swabbed" by letting down a swabbing tool which will take off the drilling fluid solid from the bore hole at the producing formation. Another method for completing the well for production is to employ acid together with a corrosion inhibitor. Still another method for completing the well for production is to apply shots of nitroglycerine. The formation may then be expanded at the producing formation by a suitable expanding instrument.
The bottom of the hole is then provided with a "gravel pack" and this may be partially done before the screen leading to the tubing is let down into the well. This tubing is smaller than the well bore including the casings and, where the pressure is not sufficient to cause the well to flow to the top, a pump cylinder and a piston are provided proximate the bottom of the well. Below the pump cylinder and piston is a screen around which gravel may be packed. The piston is provided with a pump rod which extends to the top of the well and is connected to a suitable pumping mechanism although in some cases gas and airlifts are used. The tubing passes through a cap which is called a "casing head" that is provided with a packing around the tubing. The tubing itself is also provided with a cap which may be packed around the pump rod. Extending from the tubing is the flow line which is a pipe or series of pipes leading from the tubing to the storage means.
In prior art subsea wellhead installations guidelines were used for guiding the package which defines the impacting loads so that, optimumly, the package landed axially on the wellhead. However, due to water currents, the weight of the material being landed and other external forces, axial landing on the wellhead landing surface could not be assured so that when the package did land on the wellhead, damage thereto was sustained.